I took a look and what is going on here is when you click on one of the world buttons on the main screen (the screen that shows the worlds you've played in), these buttons now lead to a "login" URL that immediately redirects to the game page. Thus, when you press the browser's "Back" button, you land on the redirect page again and the redirect kicks you immediately back to the game and so you don't actually go anywhere.
If you would like to hop around from city to city one at a time, without having to load multiple cities in multiple tabs/windows, you can still do this. Here are some different ways this can work:
- (Assuming you came from the world selection page), while in a city, (in most browsers) hold down the left mouse button on the "Back" button (or, right-click the button). A menu should appear. Select the second-most recent entry in this list, rather than the most recent. You will skip over the redirect and wind up back at the world selection page.
- Bookmark the world selection page. Then, while playing in a city, when you want to exit that city and return to the world selection, just load your bookmark.
- While in a city, open the in-game menu in the lower right corner of your screen (the gear/cog). Select the top red icon from the menu. A dialog will appear asking if you are sure you want to log out. Select the green/confirm button. NOTE: You are not actually logged out! This just returns you to the world selection screen. You remain logged in and you can pick another world from there.